SPRAY large mixing bowl with nonstick cooking spray.
HEAT marshmallows and butter in large, heavy-duty saucepan over medium-low heat, stirring frequently, for 5 to 10 minutes, until smooth. Remove from heat.
WORKING QUICKLY, stir in cereal and 2/3 cup morsels. Carefully transfer mixture into prepared mixing bowl. Cool for 5 minutes.
SPRAY hands with nonstick spray. Press marshmallow mixture into a 1/3 cup measure; then form mixture into an “egg” shape with hands. Repeat with remaining mixture to make a total of 24 eggs.
MICROWAVE remaining 1 1/4 cups morsels and vegetable shortening in medium, uncovered, microwave-safe bowl on MEDIUM-HIGH (70%) power for 45 seconds; STIR. Morsels may retain some of their original shape. If necessary, microwave at additional 10- to 15-second intervals, stirring just until morsels are melted.
DIP top of each egg into melted morsels; shake off or scrape excess against side of bowl. Place each upright in muffin pan(s). Immediately sprinkle dipped end of each egg with assorted toppings, sprinkles or sugars. Refrigerate for 10 minutes or until set. Store in airtight container at room temperature.